There are already proposed various seat belt retractors provided with such emergency locking mechanisms, but such retractors require complicated and expensive parts with an increased number of associated parts in order to render the operation of the locking mechanism precise and secure.
In the prior art of the emergency locking mechanism there are proposed and already in actual use two types of locking mechanisms, one being operable by sensing or detecting the acceleration or deceleration of the vehicle through the extracting acceleration of a webbing as disclosed in the U.S. Pat. No. 3,489,799 and the other being a locking mechanisms operable by direct detection of the acceleration or deceleration of the vehicle with the use of an inertial displacement member such as a pendulum as disclosed in the U.S. Pat. No. 3,758,044. In order to provide more reliable locking of the seat belt retractor, there is also proposed and used a retractor provided with the above-mentioned two types of locking mechanisms as disclosed in the U.S. Pat. No. 4,083,511. However, incorporation of such complicated locking mechanism results in an increased number of associated parts, thus rendering the entire retractor larger and, hence, costly.